Vehicles have been developed to perform an idle-stop when idle-stop conditions are met and then automatically restart the engine when restart conditions are met. For example, a vehicle may perform an idle-stop when a vehicle is stopped in traffic, at a light, etc., and subsequently restart the engine when motive power is requested by the driver, such as when a brake pedal is released or an accelerator pedal is depressed. By extending the period over a drive cycle during which the engine is in idle-stop, such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like.
However, the inventors have recognized an issue with such systems. Frequent switching between engine idle-stop and engine restart operations may lead to objectionable noise and audible clunks due to repeated gear meshing and un-meshing. For example, shutting down the engine during an idle-stop operation can cause the driveline of the vehicle to unwind due to elimination of the torque applied to the transmission from the engine. The unwound, or un-loaded, transmission may result in physical separation between two meshing gears due to gear lash. During a subsequent engine restart from idle-stop condition, torque is re-applied to the transmission, causing a rapid re-engagement of the various gears in the driveline and a re-winding of the driveline. This rapid re-engagement can cause increased noise, vibration, and harshness (NVH), such as audible clunks which may reduce drive feel as well as customer satisfaction. Furthermore, repeated clunks and related torsional stress may degrade transmission or driveline components (e.g., transmission gears, clutches, etc.) over time.
Thus in one example, the above issue may be at least partly addressed by a method of controlling a vehicle power-train including wheels, an engine and a transmission. The method may comprise, selectively shutting down engine operation responsive to operating conditions and without receiving an engine shutdown request from the operator, and before the engine is stopped, and while positive drive torque is still transmitted through the transmission, grounding the transmission to the vehicle. The method may further comprise, maintaining the transmission grounded until restarting the engine. Wheel brakes may be applied during the shutdown, and may be released only during the subsequent engine restart.
In one example, a vehicle power-train may include an engine, a transmission with one or more transmission clutches, a torque converter coupling the engine to the transmission, and wheels. In response to idle-stop conditions, an idle-stop operation may be initiated by deactivating the engine. This may include, for example, shutting off fuel and spark to the engine cylinders. As cylinder combustion stops, the engine starts spinning down towards rest (i.e., zero speed). As such, when the engine speed reaches zero, assuming there is no slip across the torque converter, the input torque from the engine to the transmission also reaches zero. With no torque, the driveline can unwind and the transmission gears can un-mesh due to gear lash. Herein, to reduce driveline unwinding and gear separation during the shutdown, the transmission used to maintain the transmission in a wound-up torque state, including applying and maintaining application of various clutches during an engine idle-stop, and/or grounding a transmission input and/or output to the vehicle.
In one embodiment, before the engine has stopped, and while positive drive torque is still being transmitted through the transmission, the transmission may be grounded to the vehicle, while the wheel brakes are activated. Transmission grounding and wheel brake application may be maintained until a subsequent engine restart operation. The transmission may be grounded by engaging one or more transmission clutches, and locking an engaged transmission clutch to a frame of the vehicle (such as a transmission case, chassis, etc.). By locking the engaged transmission clutch while keeping wheel brakes applied, an amount of torsion may be maintained in the driveline during the engine shutdown (that is, before a subsequent engine restart). A clutch pressure may be adjusted to adjust the engagement state of the clutch (that is, a degree of clutch engagement) to attain the desired amount of transmission torsion. Wheel brake application may be coordinated with the transmission grounding operation by adjusting a wheel brake pressure based on the clutch pressure, and further based on the amount of torsion requested. Additionally, a timing, with respect to engine speed, of the transmission grounding may also be adjusted, to adjust the amount of torsional potential energy maintained within the gear-train of the transmission during the engine idle-stop. For example, the transmission may be grounded at a higher engine speed to increase the amount of torsional potential energy maintained in the transmission during the shutdown.
During a subsequent engine restart, the engine may be selectively reactivated by turning on cylinder fuel and spark. As the engine spins up, the engaged transmission may be unlocked from the vehicle and the wheel brakes may be released. A timing, with respect to engine speed, of transmission unlocking may be adjusted based on the amount of torsion maintained in the transmission before the engine was restarted. For example, as the amount of torsion remaining in the transmission before the engine is restarted increases, the transmission may be unlocked at a higher engine speed following engine restart. Wheel brake release may be coordinated with the transmission unlocking at engine restart by decreasing brake pressure once the engine speed rises above a threshold speed.
In this way, by tying up the transmission in a wound-up torque state, torsional potential energy may be maintained in the transmission during engine shutdown and before a subsequent engine re-start. By retaining some driveline torsion in the transmission during engine shutdown, it may be possible to reduce gear tooth separation during engine shutdown, and thus the subsequent gear tooth re-engagement during engine restart. In one example, because the gear teeth do not become separated, even through substantial lash may exist, NVH during a successive engine restart may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.